1. Technical Field
The invention relates to fabrication of devices built to submicron design rules. Synchrotron-derived x-ray radiation serves for pattern delineation for small features considered unattainable by use of longer wavelength electromagnetic radiation. Pattern delineation may be 1:1 as in proximity patterning, or may be reduction as in favored forms of projection patterning. Very Large Scale Integration ("VLSI") is a prime device objective.
2. Description of the Prior Art
State-of-the-art VLSI is a 16 megabit chip with circuitry built to design rules of 0.5 .mu.m. Effort directed to further miniaturization takes the initial form of more fully utilizing resolution capability of presently-used ultraviolet ("UV") delineating radiation. "Deep" UV (.lambda.=0.3 .mu.m-0.1 .mu.m), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.25 .mu.m or slightly smaller.
At still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. An extensive effort depends on electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths.
One approach to x-ray delineation is proximity printing. In this approach, which resembles photographic contact printing, the x-ray mask is placed on top of the wafer. (To protect the fragile, costly, fine-featured mask, it is placed close to but out of contact with the wafer--thus the term "proximity"). The arrangement certainly avoids complex (reflecting) optics required for projection imaging, but is restricted to 1:1, object:image size.
Projection imaging reduces likelihood of mask damage. Reduction capability reduces cost of the now larger-feature mask. A promising version is ringfield projection with object-to-image reduction of perhaps 5:1. Ringfield makes use of an arcuate slit of high aspect ratio with all portions of the slit at constant distance from the optical axis of the lens system to avoid radially-dependent aberrations. See, co-pending U.S. application Ser. No. 07/732,559, filed Jul. 19, 1991.
Advancement in both proximity and projection x-ray lithography have been impressive. A variety of design parameters have been optimized. In proximity printing, techniques have been developed for avoiding mask damage while providing for needed close mask-to-wafer spacing. Step-and-scan is expected to yield acceptable patterns in ringfield projection. Both are believed capable of 0.1 .mu.m resolution in production.
Relatively little attention has been directed to a suitable radiation source. In projection printing it has been generally assumed that a "plasma source" would be used. Such a source depends upon a high power, pulsed laser-e.g. an yttrium aluminum garnet (YAG) laser, or an excimer laser, delivering 500-1000 watts of power to a 10 .mu.m-100 .mu.m spot--thereby heating a source material to e.g. 10.sup.6 .degree. C. to emit x-ray radiation from the resulting plasma. Such a source has many favorable characteristics. It is compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). Considerable effort is being directed to development of an x-ray plasma source.
The synchrotron represents a well-developed art for reliable x-ray emission. The synchrotron is, however, very costly, and is generally justifiable only when made available to many users at a time. It has become a fixture at national laboratories and other large institutions, where small emission space, short time rental is made available and serves a variety of sophisticated objectives.
Absent an appropriate plasma source, proximity x-ray printing, the most developed form of x-ray lithography, has used synchrotron emission. Consistent with traditional, highly demanding, scientific usage, proximity printing has been based on the usual small collection arc. Proximity x-ray printing uses its 10-20 mrad synchrotron radiation fan to produce a 1 mm-2 mm.times.40 mm line-shaped illumination field which scans the 25 mm.times.25 mm mask field. See Nuc. Inst. & Methods, 222 p. 291 (1984). Many believe that proximity x-ray printing will go into commercial use despite its many limitations--despite unavailability of image reduction; despite need for fragile membrane masks; despite unavailability of full-field imaging.
In projection lithography, a variety of considerations lead to "soft" x-ray illumination (.lambda.=100 .ANG.-200 .ANG.). In proximity printing, a shorter wavelength, e.g. 8 .ANG.-16 .ANG., is necessary to minimize resolution loss due to diffraction at feature edges on the transmission mask. In projection printing, camera optics, between the mask and the wafer, compensate for edge diffraction and permit use of longer wavelength radiation, more favorable for reflective optics. Limitations of the synchrotron source --largely cost--are little-affected by choice of the longer wavelength radiation.
Adaptation of synchrotron radiation to use in projection lithography will have to be addressed. The 1 mm-2 mm high emission fan does not lend itself either to full-field exposure (with its relatively low aspect illumination field), or to the favored ringfield approach (with its arc shaped illumination field which is designed to minimize radially-dependent aberrations--aberrations due to varying distance from the optical center of the lens system).